Summary:
**Context/Todo:**
As requested, allow IOOptions to take in an Env::IOPriority for convenience to pass down rate limiter related hint to file system level and for future interaction between RocksDB internal's rate limiting and custom file system level's rate-limiting.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9806
Test Plan: No actual code changes in RocksDB internals
Reviewed By: ajkr
Differential Revision: D35388966
Pulled By: hx235
fbshipit-source-id: 5891c97c3f9184cd221a9ab8536ce8dfa8526c08
9cd47ce554